1. Field of Invention
The present invention relates in general to the field of electronic packaging. More particularly, the present invention relates to electronic packaging that removes heat from an electronic component using thermal dissipation channels of an organic substrate.
2. Background Art
Electronic components, such a microprocessors and integrated circuits, must operate within certain specified temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Heat sinks are widely used for controlling excessive heat. Typically, heat sinks are formed with fins, pins or other similar structures to increase the surface area of the heat sink and thereby enhance heat dissipation as air passes over the heat sink. In addition, it is not uncommon for heat sinks to contain high performance structures, such as vapor chambers and/or heat pipes, to further enhance heat transfer. Heat sinks are typically formed of metals, such as copper or aluminum. More recently, graphite-based materials have been used for heat sinks because such materials offer several advantages, such as improved thermal conductivity and reduced weight.
However, the heat dissipation performance of conventional heat sinks, even when augmented with high performance structures and the use of exotic materials, may not be enough to prevent excessive heat buildup in the electronic component and/or the underlying substrate in some applications, such as in a multi-chip module that requires high power density. For example, excessive heat buildup in the module substrate of a multi-chip module can be a problem when an organic substrate, such as polyimide or other polymer(s), FR-4 (i.e., glass-epoxy laminate), etc., is used in lieu of a ceramic carrier, glass-ceramic substrate, or other conductor-carrying substrate that can withstand higher temperatures. Moreover, excessive heat buildup exacerbates another problem associated with using organic substrates, which is discussed below, relating to the mismatch in the coefficient of thermal expansion (CTE) of organic substrates relative to that of silicon chips. Hence, organic substrates are typically not used as module substrates in multi-chip modules, even though organic substrates are typically much less costly than ceramic carriers, glass-ceramic substrates, and the like.
Electronic components are generally packaged using electronic packages (i.e., modules) that include a module substrate to which one or more electronic component(s) is/are electronically connected. A single-chip module (SCM) contains a single electronic component such as a central processor unit (CPU), memory, application-specific integrated circuit (ASIC) or other integrated circuit. A multi-chip module (MCM), on the other hand, contains two or more such electronic components.
Generally, each of these electronic components takes the form of a flip-chip, which is a semiconductor chip or die having an array of spaced-apart terminals or pads on its base to provide base-down mounting of the flip-chip to the module substrate. The module substrate is typically a ceramic carrier, glass-ceramic substrate, or other conductor-carrying substrate. As mentioned above, organic substrates are generally not used as module substrates because of their inability to withstand higher temperatures and because of the typically large CTE mismatch between organic substrates and flip-chips. This is disadvantageous because, as mentioned above, organic substrates are typically much less costly than ceramic carriers, glass-ceramic substrates, and the like. Typically, ceramic carriers have a CTE (e.g., CTE=2-7 ppm/° C.) much closer to that of flip-chips (e.g., CTE 2-3 ppm/° C.) than do organic substrates (e.g., CTE=12-20 ppm/° C.). In general, the larger the base area of the flip-chip the more problematic the CTE mismatch becomes. Hence, organic substrates are typically not used as module substrates, especially for relatively large flip-chips, because of the substantial strain on the solder balls (which interconnect the flip-chip and the module substrate as discussed below) due to CTE mismatch.
Moreover, the problems associated with the use of organic substrates due to CTE mismatch are exacerbated by any excessive heat buildup in the organic substrate due to heat dissipation performance limitations of the heat sink as discussed above. Excessive heat buildup acts to magnify the strain on the solder balls due to CTE mismatch between the organic substrate and the flip-chip.
Controlled collapse chip connection (C4) solder joints (also referred to as “solder bumps”) are typically used to electrically connect the terminals or pads on the base of the flip-chip with corresponding terminals or pads on the module substrate. C4 solder joints are disposed on the base of the flip-chip in an array of minute solder balls (e.g., on the order of 100 μm diameter and 200 μm pitch). The solder balls, which are typically lead (Pb)-containing solder, are reflowed to join (i.e., electrically and mechanically) the terminals or pads on the base of the flip-chip with corresponding terminals or pads on the module substrate.
Typically, a non-conductive polymer underfill is disposed in the space between the base of the flip-chip and the module substrate and encapsulates the C4 solder joints. The C4 solder joints are embedded in this polymeric underfill and are thus protected from corrosion caused by corrosion inducing components such as moisture and carbon dioxide in the air. In addition, the polymeric underfill provides mechanical rigidity and electrical isolation.
FIG. 1 illustrates an example of a conventional multi-chip module assembly 100 that utilizes C4 solder joints. FIG. 2 is an enlarged view of the C4 solder joints of the conventional multi-chip module assembly 100. In many computer and other electronic circuit structures, an electronic module is electrically connected to a printed circuit board (PCB). For example, the conventional multi-chip module assembly 100 shown in FIGS. 1 and 2 includes capped module 105 electrically connected to a PCB 110. Generally, in connecting an electronic module to a PCB, a plurality of individual electrical contacts on the base of the electronic module must be connected to a plurality of corresponding individual electrical contacts on the PCB. Various technologies well known in the art are used to electrically connect the set of contacts on the PCB and the electronic module contacts. These technologies include land grid array (LGA), ball grid array (BGA), column grid array (CGA), pin grid array (PGA), and the like. In the illustrative example shown in FIG. 1, a LGA 115 electrically connects PCB 110 to a module substrate 120. LGA 115 may comprise, for example, conductive elements 116, such as fuzz buttons, retained in a non-conductive interposer 117.
In some cases, the module includes a cap (i.e., a capped module) which seals the electronic component(s) within the module. The module 105 shown in FIG. 1 is a capped module. In other cases, the module does not include a cap (i.e., a bare die module). In the case of a capped module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the cap, and another thermal interface between a bottom surface of the cap and a top surface of the electronic component(s). For example, as shown in FIG. 1, a heat sink 150 is attached with a thermal interface 155 between a bottom surface of heat sink 150 and a top surface of a cap 160, and another thermal interface 165 between a bottom surface of cap 160 and a top surface of each flip-chip 170. In addition, a heat spreader (not shown) may be attached to the top surface of each flip-chip 170 to expand the surface area of thermal interface 165 relative to the surface area of the flip-chip 170. The heat spreader, which is typically made of a highly thermally conductive material such as SiC, is typically adhered to the top surface of the flip-chip 170 with a thermally-conductive adhesive. Typically, a sealant 166 (e.g., a silicone adhesive such as Sylgard 577) is applied between cap 160 and module substrate 120 to seal the chip cavity 167. In the case of a bare die module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the electronic component(s). Heat sinks are attached to modules using a variety of attachment mechanisms, such as adhesives, clips, clamps, screws, bolts, barbed push-pins, load posts, and the like.
Capped module 105 includes a module substrate 120, a plurality of flip-chips 170, LGA 115, and cap 160. In addition, capped module 105 includes C4 solder joints 175 electrically connecting each flip-chip 170 to module substrate 120. As best seen in FIG. 2, capped module 110 also includes a non-conductive polymer underfill 180 which is disposed in the space between the base of each flip-chip 170 and module substrate 120 and encapsulates the C4 solder joints 175.
Module substrate 120 is typically a ceramic carrier, glass-ceramic substrate, or other conductor-carrying substrate that can withstand higher temperatures. As discussed above, organic substrates are typically not used as module substrates because of the inability of organic substrates to withstand higher temperatures and because of the typically large CTE mismatch between organic substrates and flip-chips. Moreover, the problems associated with the use of organic substrates due to CTE mismatch are exacerbated by any excessive heat buildup. This is disadvantageous because organic substrates are typically much less costly than ceramic carriers, glass-ceramic substrates, and the like.
Therefore, a need exists for an enhanced method and apparatus for removing heat from an electronic component mounted on an organic substrate.